The majority of prior art digitizers have not required cursor hold down means (i.e., a way to cause the cursor to adhere to the digitizing surface, or platen) because most such digitizers have been employed with their platens in a horizontal position. There have been instances where large platens have been mounted on tilting stands, in a manner resembling drafting tables. Some of these systems have incorporated free moving hand held cursors (that is, cursors not constrained by a mechanical linkage), and in those systems it has been necessary to provide some sort of restraining device to hold the cursor when it is not in use and the platen is tilted at a steep angle.
The present invention is associated with a digitizer having a transparent platen of modest size that tilts vertically to allow rear projection. However, the invention is applicable to any digitizer incorporating a cursor (as opposed to a stylus) whose platen can, for whatever reason, be tilted sufficiently that the cursor slides of its own accord. The invention comprises a controllable vacuum hold down for the cursor. The control can be exercised in different modes, and has many advantages over schemes involving restraining devices.
Briefly, the invention is as follows. A source of vacuum is provided. The vacuum source is responsive to a control means such that the vacuum can either be present at the cursor, or absent. The cursor is connected to the vacuum source by a flexible hose. A circular vacuum port on the underside of the cursor causes the cursor to adhere to any flat surface it is placed against, provided the vacuum is present.
In the embodiment under consideration the vacuum is high enough, in conjunction with the amount of surface area to which it is applied, that the cursor is nearly immovable when the vacuum is present. Since the cursor is relatively light in weight, this ensures that the vacuum can support an unattended cursor; even on a platen that is vertical, or even upside down (i.e., the cursor is "overhead"). In addition, the volume of space to be evacuated by the vacuum is small, so that cursor adherence appears to the operator to be instantaneous once the vacuum is applied.
The device controlling the digitizer (i.e., the device to which the digitizer supplies data) determines, by issuing commands, the mode in which the controls affecting the vacuum will operate. To begin with, the controller (the above mentioned controlling device) may specify that the vacuum be applied until the controller specifies otherwise. Or, the controller may specify that the vacuum be absent until the controller specifies otherwise. In each of these cases the operator of the digitizer cannot affect the vacuum by pressing an "A" switch (air, or vacuum, push button) located on the cursor itself.
However, the controller can issue commands that allow the "A" switch to affect the vacuum, and that indicate how the presence or absence of the vacuum is to be related to pressing the "A" switch. In one mode, called the Toggle Mode, the vacuum alternates between presence or absence, changing each time the "A" switch is pressed. In another mode, called the Normal Mode, the vacuum is continuously present except for periods of time when the "A" switch is pressed. During those intervals the vacuum will be absent.
When a digitizer having the above mentioned attributes is used in conjunction with a controller such as a programmable calculator, desktop computer, or other computer, a number of advantageous operational features may be obtained. These include:
The operator can apply cursor vacuum to cause the cursor to remain in its present location even though the cursor is unattended and the platen is tilted at a steep angle or is vertical.
The prevention of damage to the cursor if it is accidently "dropped" while the operator is digitizing in the "Normal Mode" (of vacuum control); by dropping the cursor he releases the "A" switch, which applies the vacuum so that the cursor does not fall.
The controller may be programmed to apply cursor vacuum to prevent the operator from sliding the cursor outside of a window or beyond limits established by the controller.
The controller may be programmed to apply cursor vacuum to prevent the operator from moving the cursor at a velocity higher than will allow points to be digitized with a minimum density established by the controller.
The controller may be programmed to apply cursor vacuum to enforce an interruption in the digitizing process to allow time for some internal processing to occur or until some asynchronous external event or condition occurs.
The controller may be programmed to provide the operator with tactile feedback relating to cursor position by very briefly applying cursor vacuum as the cursor is moved across imaginary grid lines established by the controller.
The controller may be programmed to lock the cursor onto a point being homed onto by the operator, once the cursor reaches that point, by applying cursor vacuum.